A DI vector core is a new addition to this program. It will support the work described in the individual sections and will be headed by Dr. Lai. DI vectors are proposed to explore the issues of altered pathogenesis via expression of viral structural proteins, cytokines and inhibitors of the immune response. The lack of an infectious cDNA clone for any coronavirus, which would allow stable expression of these factors, provides the basic rationale for DI expression. We demonstrated the high rate of coronavirus recombination; however, the use of recombinant or variant viruses with limited numbers of mutations are significantly hampered by the requirement to completely sequence the 31 kb genome to demonstrate that any alterations in pathogenesis are indeed due to the mutation under study. A centralized core to produce the DI vector pools will provide both uniformity and quality control. Experiments examining the pathogenesis of MHV-A59 using the DI vector to express the JHMV HE protein were carried out over a three year period and are a summary of over 10 separate experiments. We noted no variation in any of the four pools prepared by Dr. Zhang; however, consistency in preparation, construction of the vector RNA, monitoring the DI gene expression as well as routine titer determinations will prevent any loss of time and effort within the individual projects.